Apparatus has been developed by Spectra Inc. of Cincinnati, Ohio that successfully applies hot melts and other high viscosity materials to webs. Such apparatus generally includes a tank for the hot melt in which the hot melt is heated and maintained in liquid form, a pump submerged in the tank for delivering the hot melt to an extrusion head under a small pressure, a flat upper surface of the extrusion head with an elongated narrow opening therein, and a breast roller of hard material cooperating with the extrusion head and spaced just above the extrusion head opening, the centerline through the extrusion head opening not being coextensive with any radius of the breast roller. In operation, a web of paper or the like is fed onto the breast roller and between the breast roller and extrusion head. The distance of the breast roller from a coating thickness determination point of the extrusion head is adjusted by movement of the breast roller relative to the head thereby adjusting the thickness of coating material applied.
While the appratus developed by Spectra Inc. is extremely useful for coating webs with high viscosity materials, it has several drawbacks. One drawback is that webs of varying caliper cannot be effectively treated; low quality paper of uneven caliper when forced through the small area between the extrusion and the breast roller either is not coated properly or has a tendency to break. Of course, excessive web breakage results in an unacceptable amount of down time, and makes economic operation of the machine prohibitive.
In the past, there have been various proposals for eliminating the drawbacks of excessive web breakage in the Spectra Inc. device. One solution to the problem is to only utilize high quality paper, however, this limits the operability of the structure and increases the cost of the coated material. In another solution that has been proposed in copending commonly assigned U.S. application Ser. No. 645,619 filed Dec. 31, 1975, is to apply the hot melt material onto the surface of a roller, and use that roller as a transfer roller to apply the hot melt onto a web moving past that roller guided by a roller coated with resilient material. While this successfully avoids the web breakage problem, it does require the utilization of another roller system, which can make the capital costs higher than desirable.
According to the present invention, the web breakage problem inherent in the prior art is solved in a simple manner by providing a particular mounting for the roller or the extrusion head for movement relative to each other. Means are provided for mounting the extrusion head and breast roller for pivotal movement with respect to each other so that the size of the lead-in wedge (formed by the roller and the extrusion head forming lip) increases at a greater rate during relative movement between the extrusion head and the breast roller than the distance of the coating thickness determination portion of the extrusion head from the breast roller. This wedge increase also allows more room for air bubbles to burst into the atmosphere, thereby eliminating entrapped air and holes in the required homogeneous coating, and therefore resulting in an improved end product.
Preferably, the mounting means according to the present invention includes means for pivotally mounting the breast roller with respect to a stationary extrusion head, the pivotal movement mounting means including a lever arm operatively connected to the breast roller substantially at the center thereof, and a stationary pivot point for the lever arm located on the same side of the vertical centerline of the extrusion head opening as the lead-in wedge, and located vertically above a horizontal plane passing through the center of the breast roller. Once the breast roller and extrusion head are lapped and stoned, pivotal movement of the breast roller results in the desired lead-in wedge dimension with respect to coating thickness, and minimizes the chances of web breakage at the extrusion head opening. Alternatively, the extrusion head may be pivotally mounted as by a lever arm operatively connected to the head at a given point. In this case, a stationary pivot point for the lever arm is located on the opposite side of the vertical centerline of the extrusion head opening as the lead-in wedge, and located vertically above a horizontal plane passing through the connection point of the lever arm to the head. Depending upon the distance the extrusion head opening is from the feed within the hot melt applying machine, a tangent to the breast roller at the point of intersection of a vertical radius of the roller and the exterior surface of the roller will either be parallel to the forming lip or make an angle .beta. of about 1.degree. with the forming lip.
It is the primary object of the present invention to provide an assembly for web coating with a hot melt that minimizes web breakage at the extrusion head opening. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.